1. Field of the invention
The present invention relates to a control system for an automatic transmission of motor vehicle, and more specifically to a line pressure control for a hydraulic control mechanism of the transmission.
2. Description of the Prior Art
An automatic transmission of a motor vehicle is typically provided with a torque converter and a gear transmitting mechanism employing a planetary gear mechanism. The transmission is also provided with a plurality of frictional elements such as clutches or brakes for automatically establishing a desirable shift gear stage among the plurality of shift gear stages provided in the transmission in accordance with an engine operating condition detected by such as a vehicle speed and an acceleration stroke and the like. For this purpose, a hydraulic control circuit is provided with a plurality of solenoid valves for switching hydraulic passages in the circuit in light of a shift pattern, based on the engine operating condition obtained through a vehicle speed sensor and throttle sensor, a the like, so that the frictional elements are engaged and disengaged to perform a desirable shift operation.
Japanese Patent publication No. 54-2349 issued to Ford Motor corporation and published for opposition on Feb. 6, 1979, discloses a basic electrical hydraulic control system for an automatic transmission in which regulator valves are provided for controlling a line pressure of a hydraulic passages of circuit of the transmission, and pilot pressures of the regulator valves are controlled by solenoid valves.
Switching operation of the hydraulic passages of the hydraulic circuit must be timely done, otherwise, there might be produced a torque shock resulting from unusual change in engine and/or turbine speed.
It should be noted that there is a difference in an optimum condition for switching operation between shift-up operation and a shift-down operation of the transmission. In a shiftup operation, engine speed is dragged down because of the engaging operation of the frictional elements for establishing a proper gear stage. This means that an engine output torque greatly affects smoothness of the shift operation. It is necessary to control the switching operation in accordance with, in particular, the engine output torque. On the other hand, in a shift-down operation, the engine speed or turbine speed is more important than the engine torque in obtaining a smooth switching operation.